1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly to a liquid crystal display for accomplishing color expression.
2. Description of the Related Art
A liquid crystal display is a light-receiving device, and generally has a structure of a pair of transparent electrodes, liquid crystal sandwiched between the transparent electrodes, and a pair of polarization plates situated at front and rear of the display. As a light source is usually employed a light source emitting white lights.
In order to accomplish color expression in such a liquid crystal display, color filters of red (R), green (G) and blue (B), which are three primary colors, are prepared and white lights emitted from a light source are allowed to pass through liquid crystal or not by selectively turning the liquid crystal on or off. The white lights allowed to pass the through liquid crystal enter the color filters at their rears, and exit the color filters as red, green or blue lights. The color filters are usually arranged in a mosaic pattern or a striped pattern. For instance, a liquid crystal display employing color filters is suggested in Japanese Unexamined Patent Publications No. 60-37591 published on Feb. 26, 1985, No. 60-84580 published on May 13, 1985, and No. 3-36518 published on Feb. 18, 1991.
FIG. 1 is a cross-sectional view of a conventional liquid crystal display for accomplishing color expression.
The illustrated liquid crystal display is comprised of a backlight source 201 located at the bottom, which emits white lights, a first polarization layer 2 selecting a light directed in a predetermined direction among backlights emitted from the backlight source 201, and a second polarization layer 7 receiving a light selected by the first polarization layer 2. Between the first and second polarization layers 2 and 7 are sandwiched, in this order from bottom to top, a first transparent substrate 3 composed of transparent material such as glass, a first transparent electrode 4 composed of indium-tin-oxide (ITO), liquid crystal 5, a second transparent electrode 6, an overcoat layer 14 composed of transparent resin, color filters 202 of red, green and blue, and a second transparent substrate 9.
As illustrated in FIG. 1, red, green and blue color filters R, G, and B are regularly arranged, and light-impermeable layers 13 are formed between the color filters 202. The light-impermeable layers 13 are arranged differently in dependence on how the liquid crystal display is driven. Some of liquid crystal displays are designed to have no light-impermeable layers 13. The color filters 202 may be situated differently from FIG. 1. For instance, the color filters 202 may be situated on or above the second polarization layer 7. The first and second polarization layers 2 and 7 are usually comprised of a polarizing element such as iodine and dye which provides dichroism, a polarizing base substrate for arranging and fixating polarizing elements, such as polyvinyl alcohol, and a substrate for supporting the polarizing base substrate at either sides, such as triacetyl cellulose.
A triple channel tube is usually employed as the backlight source 201, and the color filters 202 are required to have spectral transmission factor characteristic consistent with emission spectrum of the backlight source 201.
FIG. 2 illustrates an example of spectral transmission factor of color filters and emission profile of a backlight source. In a liquid crystal display employing color filters for accomplishing color expression, an efficiency of utilizing a light is decreased, as illustrated in FIG. 2, because white lights emitted from a backlight source pass through the color filters. Hence, lights having passed through the color filters have an intensity about one-third smaller than an intensity of the white lights originally emitted from the backlight source. Thus, there is caused a problem that the lights leaving the color filters become quite weak.
In order to solve this problem, there have been suggested liquid crystal displays employing fluorescent material in place of color filters. One of such liquid crystal displays is suggested in Japanese Unexamined Patent Publication No. 8-62602 published on Mar. 8, 1996. The suggested liquid crystal display includes a backlight source emitting blue lights having a wavelength in the range of 380 nm to 420 nm, in place of a backlight source emitting white lights, and fluorescent materials excited by blue lights emitted from the backlight source and emitting lights of various colors.
FIG. 3 is a cross-sectional view of the liquid crystal display suggested in the above-mentioned Publication. The illustrated liquid crystal display includes a backlight source 1 emitting blue lights having a wavelength in the range of 380 nm to 420 nm, a first polarization layer 2, a first transparent substrate 3, a first transparent electrode 4, liquid crystal 5, a second transparent electrode 6, a second transparent substrate 9, a second polarization layer 7, a mirror 301 through which lights pass from the rear, a fluorescent material layer 8, and a third transparent substrate 302, which are deposited from bottom to top in this order.
The fluorescent material layer 8 is comprised of a first fluorescent material 10 emitting red lights, a second fluorescent material 11 emitting green lights 11, a third fluorescent material 12 emitting blue lights, light-impermeable layers 13 situated between the first to third fluorescent materials 10, 11, and 12, and an overcoat layer 14 covering the first to third fluorescent materials 10, 11 and 12 therewith. The first to third fluorescent materials 10, 11 and 12 are regularly arranged, and are excited-with the blue lights having a wavelength in the range of 380 nm to 420 nm and emitted from the backlight source 1.
The mirror 301 is provided for directing lights forwardly only. Without the mirror 301, lights having passed through the fluorescent materials 10, 11 and 12 are scattered in all directions. Lights emitted from the backlight source 1 pass through the mirror 301, and are reflected only forwardly by the mirror 301.
Since this liquid crystal display does not employ color filters, it is possible to avoid lights from being decayed due to color filters, ensuring greater brightness.
In fact, the liquid crystal display suggested in the above-mentioned Publication does not include the first and second transparent substrates 3 and 9. Hence, it is not limited in the above-mentioned Publication as to whether the second polarization layer 7 is located between the first and second transparent substrates 3 and 9, or located outside the first and second transparent substrates 3 and 9.
However, since a conventional liquid crystal display employs a polarizing plate as a polarization layer, it is necessary to position the second polarization layer 7 outside both the first and second transparent substrates 3 and 9, as illustrated in FIG. 3, and hence, it is not allowed to position the second polarization layer 7 between the first and second transparent substrates 3 and 9. Thus, above the liquid crystal 5 are located two transparent substrates 9 and 302, which would generate parallax.